Analysis of phospholipid regulation of epithelial- like architecture in the Drosophila blastoderm embryo

Abstract

Distinct apico-basal domain identity is achieved in epithelial cells via the presence of apical microvilli and basolateral surface with adherence junctions. Syncytial cells in Drosophila embryogenesis display similar onset and maintenance of asymmetric identities, which makes it an excellent and tractable system to study the role of plasma membrane (PM)-actin interactions. Previous studies in the lab have shown an onset of epithelial-like organization at cycle 11. My studies show that the apical cap expands followed by stabilization and adhesion to form the lateral furrow in each syncytial cycle. Phospholipids and their interaction with actin are known to regulate polarity and PM remodeling processes in other organisms. We found that genetically increasing the dosage of phosphatidylinositol(3,4,5)-trisphosphate (PIP3) binding tGPH (tubulin promoter-GFP-PH domain of GRP1) tag resulted in defective polygonal epithelial-like architecture and short lateral furrows. Down regulation of PI3-Kinase, which presumably lowers PIP3 levels or supposedly increasing PIP3 via its antagonist PTEN phosphatase resulted in global defects in cortical actin and desynchronized division cycles. An imbalance in either of these two phospholipids resulted in the loss of cap stabilization and short lateral furrows. Actin remodeling protein Arpc1 is enriched at the actin cap edges during expansion and formin Dia is enriched during cap adhesion and furrow extension. Cap expansion is disrupted in arp3 RNAi whereas expanded caps are visible in dia RNAi. Recruitment of adhesion and actin remodeling proteins such as DE- Cadherin, Rac1 and Dia but not Arp2/3 are lowered in PI3K and PTEN mutant embryos. These studies together show that phospholipid balance is crucial for the recruitment of Dia during cap stabilization and formation of furrow during the syncytial division cycle.